The present invention relates to an electrophotographic toner and more particularly to an electrophotographic toner to be used for an image forming apparatus such as an electrostatic copying apparatus, a laser beam printer or the like.
In the image forming apparatus above-mentioned, a developer containing an electrophotographic toner is first held around the outer periphery of a developing sleeve incorporating magnetic polarities, thereby to form a so-called magnetic brush. Then, the magnetic brush is let to come in contact with a photoreceptor on the surface of which an electrostatic latent image is being formed, so that the electrophotographic toner is electrostatically sticked to the electrostatic latent image. This causes the electrostatic latent image to be turned into a toner image. Then, the toner image is transferred to paper from the surface of the photoreceptor and fixed on the paper by fixing rollers. Thus, an image corresponding to the electrostatic latent image is formed on the paper.
As the electrophotographic toner, there may be used an electrophotographic toner as obtained by blending a fixing resin with a coloring agent such as carbon black, a charge controlling agent and the like and by pulverizing the blended body into particles having sizes in a predetermined range.
The electrophotographic toner above-mentioned may present the problem of so-called off-set such as contamination of paper at the reverse side thereof or contamination of the fixing rollers due to toner falling from the paper. In particular, when the fixing temperature is low, the toner image might not be satisfactorily fixed onto the paper (deterioration of fixing properties at a low temperature).
Of the problems above-mentioned, the deterioration of fixing properties at a low temperature occurs mainly when the molecular weight of the fixing resin contained in the electrophotographic toner is high. On the other hand, the off-set occurs mainly when the molecular weight of the fixing resin is low.
To overcome the problems above-mentioned, there have been proposed various examples of the electrophotographic toner jointly containing resin having low molecular weight and resin having high molecular weight (See, for example, Japanese Patent Unexamined Publications No. 16144/1981 and No. 3644/1985).
A conventional electrophotographic toner is not provided with sufficient heat resistance. Accordingly, when the conventional electrophotographic toner is used for a low-speed image forming apparatus in which temperature is raised to a high temperature, the toner is blocked to provoke toner blanking, a so-called rainfall phenomenon, defective cleaning and the like. The toner blanking refers to the phenomenon that giant toner particles produced as agglomerated due to blocking are caught in the space between the photoreceptor and paper to form gaps therearound, thus preventing the toner from being transferred to the paper, thereby to leave white portions on the resulting image. The "rainfall" refers to the phenomenon that toner molten and sticked to the surface of the photoreceptor drum due to blocking leave traces in the form of stripes on the resulting image. The defective cleaning refers to the phenomenon that blocked toner is sticked to the blade for cleaning the photoreceptor drum. Such defective cleaning may cause the toner blanking or "rainfall" above-mentioned.